1. Technical Field
The present disclosure relates to a high voltage transmission switch.
The disclosure particularly, but not exclusively, relates to a high voltage switch for a transmission channel for ultrasound applications and the following description is made with reference to this field of application for convenience of explanation only.
2. Description of the Related Art
As is well known, sonography or ultrasonography is a system of medical diagnostic testing that uses ultrasonic waves or ultrasounds and is based on the principle of the transmission of ultrasounds and of the emission of echo and is widely used in the internal, surgical and radiological fields.
The normally used ultrasounds are comprised between 2 and 20 MHz. The frequency is chosen by taking into consideration that higher frequencies have a greater image resolving power, but penetrate less in depth in the subject under examination.
These ultrasounds are normally generated by a piezoceramic crystal inserted in a probe maintained in direct contact with the skin of the subject with the interposition of a suitable gel (being used to eliminate the air between probe and subject's skin, allowing the ultrasounds to penetrate in the anatomic segment under examination). The same probe is able to collect a return signal or echo, which is processed by a computer and displayed on a monitor.
In particular, the ultrasounds that reach a variation point of the acoustic impedance, and thus for example an internal organ, are partially reflected, the reflected percentage conveying information about the impedance difference between the crossed tissues. It is to be noted that, the big impedance difference between a bone and a tissue being considered, with the sonography it is not possible to see behind a bone, which causes a total reflection of the ultrasounds, while air or gas zones give “shade”, causing a partial reflection of the ultrasounds.
The time employed by an ultrasonic wave for carrying out the path of going, reflection and return is provided to the computer, which calculates the depth wherefrom the echo has come, thus identifying the division surface between the crossed tissues (corresponding to the variation point of the acoustic impedance and thus to the depth wherefrom the echo comes).
Substantially, an ultrasonographer, in particular a diagnostic apparatus based on the ultrasound sonography, typically comprises three parts:                a probe comprising at least one transducer, in particular of the ultrasonic type, which transmits and receives an ultrasound signal;        an electronic system that drives the transducer for the generation of the ultrasound signal or pulse to be transmitted and receives an echo signal of return of this pulse at the probe, processing in consequence the received echo signal; and        a displaying system of a corresponding sonographic image being processed starting from the echo signal as received by the probe.        
The word “transducer” generally indicates an electric or electronic device that converts a type of energy relative to mechanical and physical magnitudes into electric signals. In a broad sense, a transducer is sometimes defined as any device that converts energy from a form to another, so that this latter can be re-processed either by men or by other machines. Many transducers are both sensors and actuators. An ultrasonic transducer usually comprises a piezoelectric crystal that is suitably biased for causing its deformation and the generation of the ultrasound signal or pulse.
Ultrasonic transducers for sonography images are usually driven by high voltage driving circuits or drivers able to generate a sinusoidal signal of variable width comprised between 3 and 200 Vpp (200 Vpp being the power supply voltage value) and frequencies in the range 1 MHz to 20 MHz, this sinusoidal signal being a control signal for corresponding generators of the ultrasound pulse to be transmitted, in particular piezoelectric crystals.
The driving of transducers for ultrasonic applications typically involves in particular the application of high voltage signals (+/−100V) with frequencies in the range of 1 to 6 MHz.
The corresponding driving circuits are thus made of components that can sustain these high voltages and that, given the frequencies at stake, can supply currents high enough to a load applied at the output, in particular an ultrasonic transducer.
This to use components with rather big sizes. These components however add high parasitic capacitances in parallel to the transducer.
Moreover, the transducer itself is also used for the receiving in a transmission channel for these ultrasound applications. Typically, an ultrasonic transducer transmits a high voltage pulse of the duration of a few μs, and receives the echo of this pulse, generated by the reflection on the organs of a subject under examination, for the duration of about 250 μs, for going back to the transmission of a new high voltage pulse. For example, a first pulse IM1 and a second pulse IM2 are transmitted with a peak to peak excursion equal, in the example shown, to 190 Vpp with reception by the transducer of corresponding echo indicated with E1 and E2, as schematically shown in FIG. 1.
The echo signal or return acoustic wave is converted into an electric wave that turns out to be a signal of some millivolts of width, signal that is then amplified by low noise amplifier circuits, connected to the transducer itself, in turn disturbed by the parasite capacity due to the high voltage components of the driving circuit of the transducer. This reduces the quality of the echo signal.
FIG. 2 schematically shows a transmission channel of an impulsive signal for an ultrasound transducer, in particular an ultrasound transducer UST, realized according to the prior art. The transmission channel is globally indicated by 20.
By way of illustration, only an output section of the transmission channel 20 has been actually shown being connected to the ultrasound transducer UST and supplying it with an impulsive signal IM generated by suitable circuitry (not shown) and already on an input terminal IN of the transmission channel 20.
Actually, the front-end portion of the driving channels of ultrasound transducers for ultrasound applications, comprises a transmission circuit able to apply a high voltage electric signal (ranging from 3 to 200 Vpp) to the piezoelectric transducer UST and an high voltage transmission switch (usually also indicated as T/R Switch) having the function of always connecting the ultrasound transducer UST with the input terminal of a receiver.
Moreover, the ultrasound transducer UST can be a piezoelectric transducer or a CMUT (Capacitive Mems Ultrasound Transducer). In any case, it works as a transmitter of acoustic waves and also as a receiver (like a microphone) by converting an acoustic signal into an electric signal.
In particular, making reference to the scheme of FIG. 2, the transmission channel 20 comprises transmission circuitry 21 being inserted between the input terminal IN and a first high voltage output terminal HVout, whereto the input impulsive signal IM is transmitted.
The transmission circuitry 21 may be, as indicated in FIG. 2, a matrix of high voltage switches (MATRIXsw) or an impulser, being able to directly generate a high voltage signal.
Furthermore, the transmission channel 20 comprises a second low voltage output terminal LVout connected to a Low Noise Amplifier 23 (LNA), for instance a transconductance cell, and a connection terminal Xdcr to the ultrasound transducer UST. Finally, the transmission channel 20 comprises at least one high voltage transmission switch 22 (TRsw) connected between the first high voltage output terminal HVout and the second low voltage output terminal LVout.
This high voltage transmission switch 22 is able to transmit an output signal to the second low voltage output terminal LVout during the receiving step of the transmission channel 20.
In particular, it is to be noted that the transmission switch 22 is a high voltage one since, during the transmission step of the transmission channel 20, a signal being on the connection terminal Xdcr, always indicated with Xdcr, is a high voltage signal even if the switch 22 is off. When this switch 22 is instead on, i.e. during the reception step of the transmission channel 20, the signal Xdcr is generally at a voltage value next to zero since the piezoelectric transducer being connected to the transmission channel 20 is detecting small return echoes of ultrasound pulse signals.
Typically, in fact, an ultrasonic transducer transmits a high voltage pulse of the duration of a few μs, and receives the echo of this pulse, generated by the reflection on the organs of a subject under examination, for the duration of about 250 μs, for going back to the transmission of a new high voltage pulse.
A high voltage transmission switch of the known type is shown for instance in FIG. 3, globally indicated by 32.
The high voltage transmission switch 32 is a passive diode-switch being inserted between the connection terminal Xdcr to the ultrasound transducer UST and the low voltage output terminal LVout and comprising at least a diode bridge 31 in turn including a first diode HVD1 and a second diode HVD2 being inserted, in series to each other, between a first or positive low voltage terminal LVP and a second or negative low voltage terminal LVN, as well as a third diode HVD3 and a fourth diode HVD4 being inserted, in series to each other, between the positive and negative low voltage terminals, LVP and LVN.
Moreover, the positive low voltage terminal LVP is connected to a positive voltage supply reference +Vpp by means of a first or positive current generator GP of a first or positive current IP and the negative low voltage terminal LVP is connected to a negative voltage supply reference −Vpp by means of a second or negative current generator GN of a second or negative current IP.
More particularly, the first and second diodes, HVD1 and HVD2, are connected to each other at a first internal circuit node XD1, in turn connected to the connection terminal Xdcr by means of a first resistor R1. In FIG. 3, a noisy impulsive signal INOISE has been indicated in order to model the transducer element. Moreover, the third and fourth diodes, HVD3 and HVD4, are connected to each other at a second internal circuit node XD2, in turn connected to the low voltage output terminal LVout. The low voltage output terminal LVout is also connected to ground GND by means of a second resistor R2 in order to model the connection to a Low Noise Amplifier (not shown). Namely, the first diode HVD1 has an anode connected to the positive low voltage terminal LVP and a cathode connected to the first internal circuit node XD1, the second diode HVD2 has an anode connected to the first internal circuit node XD1 and a cathode connected to the negative low voltage terminal LVN, the third diode HVD3 has an anode connected to the positive low voltage terminal LVP and a cathode connected to the second internal circuit node XD2 and the fourth diode HVD2 has an anode connected to the second internal circuit node XD2 and a cathode connected to the negative low voltage terminal LVN.
Usually, the first and second resistors, R1 and R2, are of a same value, for instance 100Ω. Moreover, the positive and negative current generators, GP and GN, issues corresponding currents (IP=IN) being chosen between 2 mA, 4 mA and 8 mA.
This kind of high voltage transmission switch 32 is well known in the art and is largely used in the diagnostic apparatuses based on the ultrasound sonography actually on the market. Such known apparatuses usually comprise only a limited number of channels and thus have a low resolution. Moreover, the known type of high voltage transmission switches usually comprise discrete components being directly integrated on the PC boards inside the console of the diagnostic apparatus.
The main drawback of the known solutions is tied to its high power consumption needed to bring the high voltage transmission switch on (and thus its diodes directly biased). In fact, the resistance in series being introduced by the high voltage transmission switch only depends on the biasing current.
It is thus known to raise the current flowing through the diodes in order to have a low resistance and thus a low noise level. In particular, the resistance and the biasing current of the diodes are linked by the following formula:
  Rd  =            KT      q        ·          1      Ibias      
being:
Rd the value of the resistance of a diode;
Ibias its biasing current;
K the Boltzmann constant;
T the absolute temperature; and
q the electron charge.
It should be also remarked that the high voltage transmission switch is usually connected to a terminal (Xdcr) receiving a high voltage value during a transmission phase and to a terminal (LVout) always connected to a low voltage value, being in turn connected to the input of the Low Noise Amplifier (LNA) and thus biased with a voltage between 0V and 2-3V.
Another feature of the high voltage transmission switch is that, when in its closed state, the switch should not transmit any current.